Transparent polycrystalline ceramics are known in the art. There are, however, two significant difficulties in the preparation of optically transparent ceramics: (1) randomly oriented polycrystalline anisotropic ceramics, which inherently have a different refractive index along at least two crystal axes, scatter light at each grain boundary; and (2) ceramics comprising two or more phases having different refractive indices scatter light at each phase boundary. Second phases include pores which may be present in a ceramic. Such pores contain gases which have a refractive index of about 1.0 whereas the refractive index of ceramic is typically significantly greater than 1.0 (e.g., in the range of 1.4 to 2.8).
Polycrystalline aluminum oxide is used as an optically transparent ceramic in certain applications (e.g., high pressure sodium vapor discharge lamps). The optical transparency of polycrystalline alumina, however, is limited because of its anisotropic crystal structure. An alternative to alumina is gamma-aluminum oxynitride. Gamma-aluminum oxynitride, commonly referred to as "AlON", is an AlN-Al.sub.2 O.sub.3 solid solution. In the early literature this material was sometimes referred to as "nitrogen stabilized cubic Al.sub.2 O.sub.3. " Gamma-aluminum oxynitride is more transparent than alumina because the former has a cubic crystal structure which is inherently isotropic whereas the latter has a non-cubic crystal which is inherently non-isotropic.
Synthesis of aluminum oxynitride was first reported by Yamaguchi et. al. in "Study on the Reductive Spinel--A new Spinel Formula AlN--Al.sub.2 O.sub.3 Instead of the Previous One Al.sub.3 O.sub.4 ", Bull. Chem. Soc. Jap., 32, (11), November, 1959, pp. 1264-65, wherein alumina and graphite were reacted above 1650.degree. C. in an unspecified atmosphere. The composition and structure of gamma-aluminum oxynitride were later described in more detail by Lejus in "On the Formation of High Temperature Nonstoichiometic Spinels and Derivative Phases, In Several Systems Based on Alumina and In The System Aluminum Nitride-Alumina", Temper. et Refract., Ch. 5, 1, 1964, pp. 58-95. Lejus's preparation on aluminum oxynitride included reacting aluminum nitride and alumina.
U.S. Pat. No. 4,241,000 discloses a structural ceramic material comprising sintered single phase, polycrystalline, cubic aluminum oxynitride which displays isotropic optical, thermal, and electrical properties, an infrared cutoff of about 5.2 micrometers, and which shows no chemical or physical property change after heating to 1100.degree. C. in an air atmosphere. The aluminum oxynitride ceramic was prepared by isostatically pressing a mixture of aluminum nitride and alumina powders, heating in nitrogen for 24 hours at 1200.degree. C., and then sintering in nitrogen at 1975.degree. C. for 1 hour.
Use of sintering aids, such as boron, yttrium, lanthanum compounds, or combinations thereof, to improve the optical transparency of sintered AlON is disclosed in U.S. Pat. Nos. 4,481,300, 4,520,116, 4,686,070, and 4,720,362 and in unexamined Japanese Pat. No. SHO60-191061. The latter also describes an improvement in optical transparency by using aluminum nitride powder with a mean particle size diameter of less than 2 micrometers.
Weiss et. al. in "The System Al-Mg-O-N", J. Am. Ceram. Soc., 65, (5-6), 1982, pp. C-68-69, reported that although an understanding of the phase relations and compositions of the Al-Mg-O-N system is still incomplete, gamma aluminum magnesium oxynitride may be described by the formula, ##STR1## wherein the temperature-dependent solubility limits are 0.ltoreq.x.ltoreq.1;, 0.ltoreq.y.ltoreq.1; and x+y=1. The authors did not disclose or suggest a transparent gamma-aluminum magnesium oxynitride, nor did they disclose or suggest a possible use for this ceramic material.
U.S. Pat. No. 3,026,210 discloses the use of up to 0.5 weight percent MgO as a sintering aid in the preparation of a transparent, high density, polycrystalline body of alumina having an in-line transmission of not less than 0.5 percent per millimeter thickness of the body of radiant energy of all wavelengths in the range from about 0.3 to about 6.6 micrometers and having not less than 10 percent at some specific wavelength within the range of about 0.3 to about 6.6 micrometers. The presence of higher amounts of MgO resulted in a reduction in transparency due to the increased formation of a MgAl.sub.2 O.sub.4 second phase.